[feat]: Create `<pf-v6-file-upload>` element

[markcaron]

Summary

Create a <pf-v6-file-upload> web component implementing the PatternFly v6 Simple file upload component.

Single file upload with drag-and-drop support, file type restrictions, and loading states.

Prior Art

Source	Element	Status
PFE v4	--	--
cem	--	--
cockpit	--	--
chickadee	<chickadee-file-upload>	Exists

Design

• Simple upload
• Multiple upload

Notes

• Related: <rh-file-upload>
• pf-v6-multiple-file-upload is a separate issue ([feat]: Create <pf-v6-multiple-file-upload> element #3023)

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with @patternfly/react-core FileUpload
• MAY adjust element API to leverage web platform strengths
• SHOULD maintain CSS custom property theming compatibility
• SHOULD expose useful CSS shadow parts

Element-specific considerations

• FACE: formAssociated = true, ElementInternals - submits File or file content
• value (File or string), filename, onChange event
• accept attr for file type restriction (maps to <input type="file" accept>)
• isLoading state with spinner during upload
• isReadOnly / isDisabled states
• Drag-and-drop zone: dragover, dragleave, drop events; isDragActive state
• "Browse" button triggers hidden <input type="file">
• FileUploadField sub-component: text input area for showing filename or pasting text content
• Text file mode: read file as text (FileReader.readAsText), display in textarea
• onFileInputChange, onClearClick, onTextChange callbacks
• Progressive enhancement: functional without JS for basic file input

Checklist

Implementation

• formAssociated = true
• ElementInternals for value/validity reporting
• API per .claude/ADVICE.md
• CSS uses v6 tokens, logical properties, nesting
• @summary and JSDoc on element class

Demos

• Demos match patternfly.org naming/structure
• CSS custom properties match computed styles from patternfly.org (Chrome MCP)
• Visual parity verified via Chrome MCP

Tests

• Public API covered (attributes, properties, events, slots)
• a11ySnapshot assertions
• Form submission test

Reviews

• /review-api passes
• /review-demos passes
• /review-a11y passes
• eslint and stylelint pass

Ship

• Add changeset
• PR targets staging/pfv6 (NOT main)